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Concept
Cycles per instruction
Summary
In computer architecture, cycles per instruction (aka clock cycles per instruction, clocks per instruction, or CPI) is one aspect of a processor's performance: the average number of clock cycles per instruction for a program or program fragment. It is the multiplicative inverse of instructions per cycle.
The average of Cycles Per Instruction in a given process (CPI) is defined by the following weighted average:
Where is the number of instructions for a given instruction type , is the clock-cycles for that instruction type and is the total instruction count. The summation sums over all instruction types for a given benchmarking process.
Let us assume a classic RISC pipeline, with the following five stages:
Instruction fetch cycle (IF).
Instruction decode/Register fetch cycle (ID).
Execution/Effective address cycle (EX).
Memory access (MEM).
Write-back cycle (WB).
Each stage requires one clock cycle and an instruction passes through the stages sequentially. Without pipelining, in a multi-cycle processor, a new instruction is fetched in stage 1 only after the previous instruction finishes at stage 5, therefore the number of clock cycles it takes to execute an instruction is five (CPI = 5 > 1). In this case, the processor is said to be subscalar. With pipelining, a new instruction is fetched every clock cycle by exploiting instruction-level parallelism, therefore, since one could theoretically have five instructions in the five pipeline stages at once (one instruction per stage), a different instruction would complete stage 5 in every clock cycle and on average the number of clock cycles it takes to execute an instruction is 1 (CPI = 1). In this case, the processor is said to be scalar.
With a single-execution-unit processor, the best CPI attainable is 1. However, with a multiple-execution-unit processor, one may achieve even better CPI values (CPI < 1). In this case, the processor is said to be superscalar. To get better CPI values without pipelining, the number of execution units must be greater than the number of stages.
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